Chapter 4 - Imperfections in Solids

What is screw dislocation?

1. A spiral planar ramp resulting from shear deformation 2. The ramp will always shift the lattice structure downwards 3. The burger's vector is parallel to the dislocation line

What is edge dislocation?

1. An extra half-plane of atoms is inserted into a crystal structure 2. Represented as an upsidedown 'T' 3. The burger's vector is perpendicular to the dislocation line

What are the conditions for formation of substitutional solid solutions (W. Hume - Rothery Rules)

1. Difference of atomic radius 'r' must be less than 15% 2. Must have similar electronegativities (proximity in the periodic table) 3. Same crystal structure for pure metals (EX: BCC and BCC, FCC and FCC, FCC and HCP) 4. Valences, where, all else being equal, a host metal will have a greater tendency to dissolve a metal of higher valence than one of lower valence

What type of defect is copper in nickel, and what is the desired benefit?

1. Impurity 2. Resistivity increases

What are the 2 steps to solidification?

1. Nuclei form in the liquid, and then turn into crystals that are also in liquid 2. Crystals grow until their grain boundaries meet each other, which means the material is a polycrystal

What are the two outcomes of B atoms in a solid of A atoms in metals?

1. Random distribution of a solid solution of B in A. Can be further divided into substitutional solid solutions and interstitial solid solutions 2. Compound solid solution of B in A, plus particles of a new phase. This occurs for a larger concentration of b, where the second phase particle has a different composition and different structure

What are grain boundaries?

1. The regions between grains, where an angle of misalignment exists between the atoms of adjacent grains. 2. There exists a slight atomic disorder at grain boundaries in the form of high atomic mobility and high chemical reactivity. 3. Thus, atoms along grain boundaries have more energy because they are less stable 4. A material will always react to a chemical at its grain boundaries first

When are polarized optical microscopes used?

1. They are used to increase the contrast of metallic specimens 2. Used for transparent samples such as polymers

What is a compound?

1. When enough impurity atoms are present in certain ratios with the host atoms, a new phase can occur 2. The new phase will have a different alignment/structure/composition within the structure of the compound

What are the values of the Boltzmann's constant?

1.38 x 10^(-23) J/(atom-K) 8.62 x 10^(-5) eV/(atom-K)

What is the average equilibrium concentration of metals below their melting point temperature?

10^(-4), meaning that one lattice site out of 10,000 sites will be a vacancy

How to find the number of grains per square inch at 100X magnification?

2^(G-1) Where G is the ASTM grain size number

What is electron microscopy?

A beam of electrons is focused using magnetic lenses to achieve a magnification as high as 1000000X, which is the atomic scale. Brighter images reflect more electrons

What are mixed dislocations?

A combination of edge and screw dislocations on the same crystal structure and result in zig-zag lines

What is dislocation?

A linear defect (1D) that is responsible for the deformation of a material caused by the misalignment of atoms

What are bulk defects?

A macroscopic 3D imperfection such as cracks, pitting, etc.

What are scanning tunneling microscopy (STM)?

A microprobe that tapers to a single atom at its tip to image surface atoms. The surface atoms can be moved by pushing them with the microprobe

What is a solid solution?

A solid solution of two metals where the structure homogeneous throughout the substance and exists as one phase

What is the burgers vector?

A vector that gives the magnitude and direction of the lattice distortion caused by a dislocation

What is the best resolution for optical microscopes?

About 0.1 um (micrometer), which is 100 nm (nanometer)

What is the different between solid solutions and alloys?

Alloys can form solid solutions, but there can be regions where different phases exist in the same structure (compounds)

Which is more soluble in Zinc (Zn): Aluminum (Al), or Silver (Ag)?

Aluminum (Al)

What does ASTM stand for?

American society of testing and materials

What is the relationship between vacancies and temperature?

An increase in temperature causes an exponential increase in the number of vacancies, where each lattice site is a potential vacancy

How do dislocations cause linear defects?

As dislocations move due to stress, plastic deformation results from the extra plane of atoms exiting the crystal structure. Each "step" in the specimen corresponds to dislocation that have moved to the crystal surface

Why do grains appear as different colors from a cast?

Because the crystal alignment of the grains reflect light differently

What is grain refiner?

Chemical that is added to casts to make smaller, more uniform equiaxed grains to improve the quality of the cast

How are grains seen by an optical microscope?

Crystallographic planes appear brighter the more light enters the microscope, and darker the less light enters the microscope

What determines the strength of a material?

Dislocation can interact with each other and prevent each other from moving. If these dislocations are stopped from moving, then the strength of the material increases

What are types of linear defects (1-dimensional)?

Dislocations

What are two types of dislocations?

Edge dislocations and screw dislocations

What are types of interfacial defects (2-dimensional)?

External surfaces, grain boundaries, phase boundaries, twin boundaries, stacking faults

How are grain boundaries visible in an optical microscope?

Grain boundaries are seen as dark lines because they react to the acid in etching more so than the other regions. Since they are etched, the groves scatter the light away from the microscope, giving the appearance of black lines

What are columnar grains?

Grains that are elongated in one direction due to heat transfer. Columnar grains elongate in the direction of heat transfer

What are equiaxed grains?

Grains that are small, spherical, and are roughly the same dimension in all directions

Which crystal structure does tin cry happen?

HCP

What are catalysts?

Increase the rate of a chemical reaction without being consumed

What is the most important type of defect?

Linear defects

Do casted objects provide good or mediocre properties?

Mediocre properties because the grains in the polycrystalline structure are of different size and alignment

What are twin boundaries/planes?

Mirror reflections of atom positions of one side of the twin plane to the other side. As these planes move, they change the shape within the material, which can cause noise called mechanical twinning (tin cry)

Where do catalytic reactions occur?

Normally at the surface defect sites because the atoms at these sites have higher energy and are thus more reactive (EX: Catalytic Converter)

What are stacking faults?

Occur when there is an error in the planar stacking sequence such as ABCABABC, AABC, etc.

How does shrinkage occur?

Shrinkage occurs when a material transitions from a liquid to a solid

What is the atomic symbol for tin?

Sn

What are interstitial positions?

The empty space between atoms when they are arranged in a lattice structure. These empty spaces exist where the corners of the atoms would be if they were cubes instead of spheres

What is activation energy?

The energy required to form a vacancy

Which metal is the solvent and solute?

The host atoms are the solvent, and the impurity atoms are the solute

What is a dislocation line?

The line along which a dislocation/distortion occurs

What is the defect concentration?

The number of vacancies divided by the total number of lattice sites

What is solidification?

The result of casting of molten material, where the process starts with a molten material that is all liquid

How to prepare a sample for optical microscopy?

The sample must have a smooth surface (polished) and etched (by acid) to expose crystals and reflect light into the microscope

How to determine the activation energy from a graph?

The slope of the ln(N_v/N) and 1/T curve is -Q/k. Thus, multiplying the slope by -k will give the activation energy

How do dislocations appear on electron micrographs?

They appear as dark lines

True of False: The angle of misalignment is the same for most atoms along a grain boundary

True

True or false: Defects and imperfections can be desirable in materials?

True (EX: impurities in silicon can aid in semiconductor usage)

True or false: Iron is a polymorphic metal?

True because it can exist as FCC and BCC structures, where more alloys are possible in the BCC form due to larger interstitial sites

What is optical microscopy?

Uses light and optical mirrors to give an image up to 2000X magnification. Mainly used for metallic samples

What are types of point defects (0-dimensional)?

Vacancies, interstitial atoms, and substitutional impurity atoms

What are vacancies?

When an atom is absent from its placement in a lattice structure, which causes an inwards distortion of the planes. The distortion of the planes goes away the further away from the vacancy site the planes are analyzed

What are self-interstitials?

When original atoms are positioned in the interstitial positions between atoms in the lattice structure. There exists an outwards distortion, which causes high amounts of stress in the immediate area of the distortion

What is the surface island of atoms?

When the equilibrium vacancy concentration increases, the vacant atoms diffuse to the surface of the crystal and form an island. When the specimen cools, the island atoms will fill their vacancies to maintain equilibrium vacancy concentration

Are defects necessary?

Yes, because defects must exist to achieve equilibrium